Any smaller motor like the RB30 in a bigger vehicle like a Pat should look at boosted induction to compensate for lack or power/torque needed to really move and control the weight.

The RB25DET head is almost a bolt-on option and the manifolds for them can be stolen off a Skyline or VL commo. The multi-valve will allow better breathing but also way high revs, all the way to 9K if your bottom end held.

A couple of options lay for you but the current and tired donk you have now is not one of them. Engine conversion is a good way to get more cubes along with a strong engine to start with. You will already know the common choices for conversions like the LS motors and the likes but you also can go the older V8's like the good old 5.0L out of the Holdens that produce great bottom end and a even better exhaust note. They respond well to PD blowers as well. Another option is in this field is to swap in a TB42 and do her up. Strong, straight 6 that also respond well to turbos and running straight gas would allow a good matching. Great power with still good economy when needed.

Otherwise, source another RB30 block and start a fresh build on it with all the trimmings, then do a swap at a time that suits you rather than being off the roads for weeks or months while you rebuild the sad one if it lets go.